Avoid copying a shared_ptr on SelectionRoute
Avoid copying a shared_ptr on SelectionRoute

Summary:
We were copying a shared_ptr in SelectionRoute). Returning a reference now to
avoid the copy/atomic operation.

Reviewed By: glamtechie

Differential Revision: D15487709

fbshipit-source-id: 5bb3c2b99ce72e7ca7a514c5eaf487583e899d9b

facebook/mcrouter,facebook/mcrouter,facebook/mcrouter,facebook/mcrouter